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FSTB Capillary tube thermostats

FSTB Capillary tube thermostats

Electromechanical temperature controls with capillary tube phial

FSTB capillary tube thermostats are used all over the world to control and limit temperatures (temperature range from 0°C to +400°C)
FSTB products are manufactured professionally in accordance with latest technology
They operate safely and reliably, provided that specification required is clear at the start of the project.
FSTB supply temperature controls, sensors and limiters with phial and with capillary tube length according to customer specification for applications such as:


  • Boilers
  • Night storage heaters
  • Heat exchangers
  • Water heaters
  • Laboratory equipment
  • Ovens of domestic cookers
  • Washing machines
  • Dishwashers
  • Deep fat fryers
  • High pressure jet washers

Principle of operation
The pressurised system which operates according to the principle of expansion of liquid is the heart of a hydraulic thermostat.
It comprises phial, capillary tube and diaphragm.
Following an increase in temperature at the phial, the liquid expands in the capillary tube, causing the diaphragm to move. This movement activates a snap-action switch, causing opening or closing of switch contacts.
It is clear that a high degree of precision is required in manufacture, as a change in temperature of 
1°K can cause a movement of the diaphragm between 0,002mm - 0,01mm.

It is therefore absolutely essential that the phial and capillary tube should not be squashed or otherwise damaged!
The thermostats are calibrated at room temperature 20°C. If during operation the capillary tube and diaphragm are subjected to a higher temperature the switching point will be changed. If the ambient temperature (TU) is known in advance this can be taken into account during the calibration process. Alternatively it is possible to compensate for the influence of ambient temperature upon the liquid at the diaphragm by fitting a bi-metal device.

WQB Series Capillary thermostat(Thermal Protector)

WYG Series Capillary thermostat(Liquid Expansion Thermostat)

WYC Series Capillary thermostat(Control+Protection)



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